Cryptology ePrint Archive: Report 2007/339

On the security of a class of image encryption schemes

Chengqing Li and Guanrong Chen

Abstract: Recently four chaos-based image encryption schemes were proposed.
Essentially, the four schemes can be classified into one class,
which is composed of two basic parts: permutation of position and
diffusion of pixel value with the same cipher-text feedback
function. The operations involved in the two basic parts are
determined by a pseudo random number sequence (PRNS) generated from
iterating a chaotic dynamic system. According to the security
requirement, the two basic parts are performed alternatively for
some rounds. Although the designers claimed that the schemes are of
high quality, we found the following security problems: 1) the
schemes are not sensitive to the changes of plain-images; 2) the
schemes are not sensitive to the changes of the key streams
generated by any secret key; 3) there exists a serious flaw of the
diffusion function; 4) the schemes can be broken with no more than
$\lceil\log_L(MN)\rceil+3$ chosen-images when the iteration number
is equal to one, where $MN$ is the size of the plain-image and $L$
is the number of different pixel values; 5) the cryptanalysis on one
scheme proposed by another research group is questionable.